The present invention relates to composite resin articles or assemblies and to methods of producing composite resin articles or assemblies wherein at least one component or member is separately preformed and subsequently molded with and to another component or member. The articles of the present invention are particularly adapted to uses wherein one portion of the article requires functional properties that are not required by the other portions. The present invention is particularly adapted to the production of composite friction assemblies, such as brake or clutch assemblies.
Phenolic resin molding compositions are well known in the art. Generally, such compositions consist of a phenol-aldehyde resin blended with various filler materials. The molding compositions are prepared by blending a one- or two-step phenol-aldehyde resin with filler materials. The mixture may then be worked, i.e., heated to from about 150.degree. to about 200.degree. C. while being rolled between hot rolls, allowed to cool, ground and sized to produce the resin product. Alternatively, the mixture may be fed into a heated screw extruder and extruded through a narrow orifice and the product comminuted to produce a nodular molding resin composition.
The phenol-aldehyde resin may be made from phenols, such as phenol, m-cresol, m,p-cresol mixtures, cresylic acid, mixtures of phenol and cresylic acid, xylenol, resorcinol, bisphenol A, or any other phenol which will form thermosetting resins with aldehydes. Suitable aldehydes, for example, are formaldehyde, acetaldehyde, benzaldehyde, furfural, propionaldehyde, glyoxal, acrolein, and crotonaldehyde. The preferred thermosetting resin is phenol-formaldehyde resin.
The phenol-aldehyde resins useful in the present invention may be one- or two-stage resins. If less than one mole of the aldehyde is reacted per mole of phenol, the resin is commonly called a novolac, or two-stage resin. A novolac mixture is generally further processed by grinding and blending with an external cross-linking agent, such as hexamethylenetetramine, to produce a thermosettable resin composition that becomes infusible at elevated temperatures. Generally, a range of aldehyde to phenol in a novolac resin is between about 0.5 and about 0.9 mole of aldehyde per mole of phenol, and, more preferably, the range is between about 0.6 and about 0.8. If more than one mole of aldehyde per mole of phenol is utilized, a one-stage, or resole, resin is produced. Such mixtures become infusible by exposure to elevated temperatures. Generally, the mole ratio of aldehyde to phenol in this type of resin is between about 1.1 to about 3.0, and, more preferably, between about 1.5 and about 2.5.
The thermosettable resin compositions suited to form the matrix of the assemblies of the present invention may be either one-stage or two-stage resins, or mixtures thereof, the only criteria being that the resins utilized in separate members be compatible, e.g., that they may be commonly cured or thermoset under relatively the same conditions of temperature and time.
Filler materials utilized in the present resin compositions may be organic or inorganic. Filler materials are added, primarily, to enhance the properties of the final molded product, and, secondarily, to utilize a less expensive material in place of the more expensive resin material. Examples of inorganic filler materials are metals and metal oxides, asbestos, clay, silica, chopped fiber glass, calcium carbonate, minerals, e.g., wollastonite, talc and quartz, coal, mica, and carbon black. Examples of organic filler materials are rubber, wood flour, cloth fibers, rag pulp, wool and cotton flock. The characteristics of the final cured product, for example, structural strength, electrical conductivity, moisture resistance, heat resistance, wear resistance, thermal expansion and conductivity, may be advantageously modified or improved by the choice and amounts of filler materials.
Although the choice of filler materials is broad, the amount of filler materials usually included in resin molding compositions, those compositions useful to produce molded articles of complex shapes, is limited due to the increased viscosity (decreased flowability) of the composition as the amount of filler material is increased. Usually, the total amount of filler material in molding compositions ranges from about 20 to about 70 percent by weight of the composition. The maximum amounts of filler material included in molding compositions may be increased to about 80 percent by the use of processing aids which lower the viscosity of the composition. However, compositions with a filler material content above about 70 percent are usually extruded and molded into complex shapes only with difficulty.
Resin compositions having a loading of 90 percent or more by weight are useful in many applications, the amount of filler material being limited by the cohesiveness and structural strength desired in the final cured product. Such compositions are not considered moldable in the same sense that the molding compositions discussed above are moldable to produce articles of complex shape. However, such compositions may be formed or molded into simple shapes using straight, positive molds and applying the forming pressure directly to the composition. The friction member of the present invention contains a relatively high loading of filler material, from about 70 to about 90 percent by weight, and is suitably formed in this manner. The support member of the present invention is suitably fabricated of conventional molding compositions to facilitate the forming of support members with more complex shapes.
The phenol-aldehyde resin component generally comprises from about 10 to about 60 percent by weight of the composition. Generally, conventional molding compositions utilize a range of from about 20 to about 50 percent by weight. Usually, less than about 10 percent by weight resin does not yield a product having a cohesiveness required for most applications. A product having a resin content of over about 60 percent by weight usually does not have acceptable physical properties, e.g., hardness, thermal conductivity or wear, and is not economically competitive with compositions containing larger amounts of filler materials.
In practice, the molder usually selects the most appropriate molding composition for the article and utilizes that composition to mold the entire article. The present invention allows the molder to utilize separate resin compositions for separate components of the article and subsequently cure the components together to form a composite article.